How to Find When a Bug Was Introduced in Git Using Git Bisect

Use git bisect to find when a bug was introduced by performing a binary search through your commit history. Mark a known good commit and a bad commit, then test each step until Git pinpoints the exact commit that caused the bug.

Syntax

The basic git bisect workflow involves these commands:

• git bisect start - Begin the bisect session.
• git bisect bad - Mark the current commit as bad (where the bug exists).
• git bisect good <commit> - Mark a known good commit where the bug did not exist.
• Test the commit Git checks out and mark it as good or bad accordingly.
• git bisect reset - End the bisect session and return to the original branch.

bash

git bisect start
# Mark current commit as bad
git bisect bad
# Mark known good commit
git bisect good <commit>
# Repeat testing and marking commits as good or bad
# After bisect finishes
git bisect reset

Example

This example shows how to find the commit that introduced a bug using git bisect. Suppose the bug is present in the current commit, but not in commit abc1234.

bash

git bisect start
# Current commit has the bug
git bisect bad
# Known good commit without the bug
git bisect good abc1234

# Git will checkout a commit in the middle
# Test if bug is present
# If bug is present:
git bisect bad
# If bug is not present:
git bisect good

# Repeat testing until Git finds the first bad commit

# When done, reset to original branch
git bisect reset

Output

Bisecting: 3 revisions left to test after this (roughly 2 steps)
[def5678] Fix typo in README
Bisecting: 1 revision left to test after this (roughly 1 step)
[ghi9012] Add new feature
First bad commit found: [ghi9012] Add new feature

Common Pitfalls

• Not marking commits correctly: Always mark commits as good or bad based on testing results to guide Git.
• Using an incorrect good commit: The good commit must be before the bug was introduced, or bisect will fail.
• Not resetting after bisect: Always run git bisect reset to return to your original branch.
• Automating tests: For large projects, use git bisect run <script> to automate testing.

bash

## Wrong way: Marking bad commit as good

git bisect start
git bisect bad
git bisect good <commit_with_bug>
# This will cause bisect to fail

## Right way: Mark a commit without the bug as good

git bisect start
git bisect bad
git bisect good <commit_without_bug>

Quick Reference

Command	Description
git bisect start	Start bisect session
git bisect bad	Mark current commit as bad (bug present)
git bisect good	Mark known good commit (bug absent)
git bisect reset	End bisect and return to original branch
git bisect run